As makers of running fuel, you'd be amazed at the amount of time we spend advising new customers at shows not to take our bars out with them on race day. This isn't because we aren't super confident in our product but because as runners we know how important it is to try everything in training first.
This might sound like common sense and we all know that we need to get training runs in if we want to do well at the race but it can be easy to forget that we need to do the same for our stomachs.
And just as you wouldn't train for a marathon just by swimming, you need to train your stomach with some specificity too. This is why it's so important to develop a running fuel routine as early as possible. Not only will this allow you to discover what works well for you and what doesn't, but it will also allow your gut to get the most out of those nutrients as efficiently as possible.
There has even been a a recent study in Research in Sports Medicine, which has shown that training your tummy leading up to a 4 hour plus event can decrease the dreaded belly drama during the race. The scientists recommend consuming 90 grams per hour of carbohydrate while running at roughly 60 percent effort for a minimum of two weeks prior to the race.